1. Field of the Invention
This invention relates to a wax emulsion formulation, and more specifically, to a formulation for imparting water repellency characteristics to products made from gypsum slurries, such as gypsum wallboards or panels.
2. Description of the Prior Art
The production of gypsum based materials normally requires a slurry of calcium sulfate hemihydrate with water. The mixture is allowed to hydrate, or set to the harder calcium sulfate dihydrate, and the excess water is removed by drying through conventional methods. Gypsum products such as wall board panels are produced from a hardened gypsum core sandwiched between paper liners, with the liners forming the outer surfaces of the board. To overcome problems of water absorption by the hygroscopic gypsum, which may lead to weakening of the wall board, it is known to add hydrophobic components into the gypsum slurry prior to making the wallboards. Such hydrophobic components, such as asphalt, waxes etc, must be provided in the form of an emulsion so as to be miscible in the aqueous gypsum slurry. These emulsions provide the gypsum wallboards with water repellent characteristics. Examples of water repellent emulsions for gypsum slurries are provided in U.S. Pat. Nos. 5,437,722; 5,098,943; and 4,748,196.
Some of the problems associated with the known wax emulsions for gypsum slurries are as follows:
1. Wax-asphalt emulsions are generally unstable. That is, the emulsion tends to separate over time into a liquid phase and a solid crust floating on top. The crust is not easily re-emulsified with mixing and can form lumps in the mix. When the board is produced, the un-emulsified asphalt component bleeds out of the set gypsum core and shows up as black spots on the paper liners.
2. Wax-asphalt emulsions are dark because of the asphalt component, which makes the gypsum core in the final board product off-colour.
3. Asphalt is generally derived from crude petroleum refining processes, which can vary in properties and, therefore, increase the variability in emulsion usage, end-product quality and wallboard manufacturing.
4. Other wax substitutes, such as montan wax or lignite, while providing good water repellency, are known to cause a build-up of dark or black sludge in process tanks used for emulsification.
5. Many wax emulsions based on montan wax or other natural or synthetic ester waxes, while they are more readily re-mixed than asphalt wax emulsions, still tend to separate during storage. This separation or “creaming” effect results in inconsistent performance characteristics.
6. Wax emulsions based on montan wax exhibit deficiencies related to the crude nature of the montan wax, which contains a variety of components. The composition of montan wax is also known to vary from one geographic region to another. The variability of the components results in discoloration of the resulting emulsion and variable emulsion quality.
Applicant's prior U.S. Pat. No. 5,968,237, teaches an aqueous wax emulsion specifically comprising hydrocarbon wax, an alkyl acid and/or alkyl ester, and a polyvinyl alcohol, for addition to a gypsum slurry. The slurry may be used in the production of wall boards and panels. The emulsion composition is intended to provide water repellency to the wall board formed without adversely affecting the stability of the gypsum slurry.
The water repellent emulsion composition proposed in U.S. Pat. No. 4,748,196 comprises a wax and an olefin-maleic anhydride copolymer in the presence of a water-soluble alkaline compound. The olefin is selected from a group consisting of an adduct or copolymer of an α-olefin and the wax component includes paraffin wax, montan wax, and a low melting point polyolefin wax. The composition is intended to provide good water repellency without affecting the stability of the emulsion however, the use of montan wax results in some discoloration and variability of the emulsion. In addition, an α-olefin is generally a single medium-length, even-numbered, unbranched aliphatic chain having a carbon to carbon double bond located at the α end only for the binding site of the anhydride.
The present invention seeks to provide an improved wax emulsion which overcomes some of the deficiencies of the prior art emulsions.